1. Field of the Invention
This invention relates to a fittable pipette tip consisting of a vessel which is designed to fit a particularly conical fitting head of a pipette and which may be conical and has a fitting top opening and a bottom opening for receiving and discharging a pipette, wherein a sealing portion with which the pipette tip is adapted to contact the fitting head is spaced from the top end of the pipette tip and is formed with a smooth inside seating surface and has a smaller wall thickness than other portions of the pipette tip so that said sealing portion of the pipette tip is yieldable for adaptation to said fitting head.
The invention also relates to such a pipette tip which consists of a vessel which is conical at least in part and in which the sealing portion is spaced from the fitting opening provided at the top end of the pipette tip and has a smooth seating surface.
Alternatively, the housing may have a cylindrical bottom end portion.
The reference to a smooth seating surface is also applicable to a roughened surface. The provision of a slightly profiled seating surface for making an improved seal with the conical fitting head is included. But a smooth seating surface will generally be preferred.
The fitting head of a pipette may consist of a cone, which is formed with a through passage and is inserted into the conical wall portion of the pipette tip. Such fitting cone may alternally be provided on and protrude outwardly from a tip portion of the pipette and may constitute a protruding ring so that the fitted pipette tip will be in sealing contact with the pipette only at said cone.
To provide a sealing seat, the outwardly protruding fitting head may have a crowned surface in its annular seating portion. This will ensure a sealing contact, particularly with an elastic and yieldable seating surface of the pipette tip. The elasticity will ensure that the pipette tip will adapt itself to the annular seat portion of the fitting head in sealing contact therewith.
2. Description of the Prior Art
Pipette tips having a conical seating surface are known, e.g., from German patent specification No. 10 90 449. They are intended to be fitted on a fitting cone which has a suitable taper throughout its length. Whereas the fittable tip may be yieldable, this will permit only a small adaptation of the conical socket of the tip to the fitting cone of the pipette and leaks have often occurred in the practical use of such assemblies.
German patent specification No. 12 91 142 discloses a pipetting device comprising a fitted pipette tip which has at its top a stabilizing protruding rim. The wall thickness of the pipette tip is constant from top to bottom. In its top portion, which comprises the sealing portion, the pipette tip is stiffened by axial ribs on the wall. A stiffening is desired particularly at the seating surface. But that measure also has not resulted in a reliable seal.
The same disadvantage is inherent in the design disclosed in East German patent specification No. 50,016. In accordance therewith the fittable vessel has a conical seat that is to be fitted on a correspondingly tapered fitting head of the pipette. The rim around the top opening is stiffened by a bead.
A stiffening of the seating portion of a fittable pipette tip has been disclosed in U.S. Pat. No. 3,732,734, where the fitting portion is reinforced by a peripheral annular outer wall portion which protrudes outwardly to form a shoulder. Whereas annular ribs are provided on the inside surface, there are no thinner wall portions which would render the wall flexible but the ribs constitute only peripheral annular sealing strips and at their base merge into a virtually undeformable wall portion of the pipette tip. Even adjacent to said ribs said wall portion has a basically conical shape just as the fitting head of the pipette.
Such ribs do not ensure a reliable fitting and do not constitute a virtually smooth seating surface. That known pipette tip has, e.g., only a peripheral flange at the top whereas elsewhere the wall thickness is constant substantially throughout the length of the pipette tip. The wall is possibly reduced in thickness only adjacent to the dispensing bottom opening.
U.S. Pat. No. 4,349,109 discloses a pipette tip which is composed of a plurality of sections. The uppermost of said sections is frustoconical and its conical inside surface has a taper of, e.g., 3 degrees for an interference fit with a mating surface of a pipette. In that section, which is formed with the seating surface for sealing contact, the pipette tip has the largest wall thickness and for this reason said section is particularly unyieldable. From said seating surface a downwardly flaring shoulder surface extends to the bottom wall section of the pipette tip. That bottom wall section has an inside surface which is inwardly offset from the seating surface of the top section so that the pipette tip can be fitted also on pipettes which have a smaller volume. But owing to the outwardly protruding wall of the top section the bottom section is particularly strong and unyieldable at its top rim. For this reason the pipette tip is not adaptable and has no overlapping profiled inside surfaces because the inside surfaces are offset from each other transversely to a center line.
A fittable pipette tip of the kind described hereinbefore has been disclosed in German patent specification 25 26 296. That known pipette tip has in its sealing portion a smaller wall thickness for adaptation to the conical fitting head. For this purpose the pipette tip must be made of a material, particularly plastic, which is particularly elastic and at least flexible and preferably tends to restore itself when it has been expanded. A rubber material has been mentioned as well as a commercially available plastic known as polypropylene.
From the last-mentioned publication it is also apparent that the yieldability and adaptability provided to improve the fit is intended to compensate dimensional inaccuracies of the pipette tip and of the fitting head of the pipette so that larger manufacturing tolerances will be permitted and an improved seal will be obtained. The forces which must be exerted to fit the pipette tip on the pipette and to remove the pipette tip from the pipette should be reduced.
The design of the known pipette tip with a portion having a smaller wall thickness has proved satisfactory. In that portion the wall thickness is reduced by the provision of an annular peripheral recess or of grooves in the outside surface. The portion which is recessed on the outside may have a smaller wall thickness at the top rim of the sealing portion than at the bottom end of the pipette. An external annular shoulder surface may be formed at the top end of the sealing portion and the wall thickness of the pipette tip may gradually decrease toward the top in the sealing portion.
Such a recess in the outside surface will be preferred because such pipette tips can conveniently be made by suitable injection molds. But it has been found in practice that such pipette tips can be removed from the mold only with difficulty unless undercuts are avoided. Particularly with slender pipette tips having only a small taper it is very difficult to sufficiently reduce the wall thickness in the sealing portion and to obtain a constant reduced wall thickness throughout a defined sealing zone. The wall thickness usually varies in the sealing portion. Besides, in such pipette tips the sealing portion cannot easily be detected with the eye so that the user may be uncertain how the pipette tip is to be properly handled.